Uniphase CCDs are well known. See for example U.S. Pat. No. 4,229,752 which issued on Oct. 21, 1980. A uniphase CCD generally employs a continuous gate electrode over the signal channel made up of a series of cells and the mobile charge packets are transported between successive cells of the channel by a single-phase clock signal periodically applied to the continuous gate electrode. In a buried-channel CCD, the mobile charge is transported in an induced channel within the bulk of the semiconductor substrate rather than along the surface for improving the charge transfer efficiency.
In a uniphase CCD, each of the plurality of cells which forms the signal channel includes four sections, a first storage section, a first transfer section, a second storage section and a second transfer section. To ensure the desired unilateral transport of the charge packets along each cell, its four sections are designed to form potential wells of varying depth under control of different concentrations of dopants. Moreover, a portion of each cell needs to include an inversion layer at its interface with the overlying silicon oxide surface to function as a virtual electrode, shielding the underlying cell sections from any gate-induced change in potential.
As is known, for a high density CCD it is desirable to keep the length along the channel of each cell short. Moreover, for optimizing the storage efficiency of each cell, it is desirable to maximize the ratio of the length of the storage sections to the length of the transfer sections of each cell. The invention is useful for increasing this ratio.
Generally, the packing density of the cells in a CCD is limited by the resolution and alignment capabilities of the photolithographic apparatus used in patterning the various masking layers conventionally used in the processing of the CCD. The accuracy of alignment of successive masks employed also serves as a limiting factor. In the past, these factors have served to limit the optimum design that can be readily achieved in practical manufacture.